Multimodulation transmitter

ABSTRACT

The present invention discloses a double TRU (Transceiver Unit) ( 45 ). The output signals from the power amplifiers ( 64, 84 ) are combined to one common output signal provided to an antenna arrangement ( 91 ). A DSP (Digital Signal Processor) ( 52, 72 ) of each TRU ( 50, 70 ) comprises means for a constant-envelope modulation scheme ( 54, 74 ) and a non-constant envelope scheme ( 53, 73 ). The DSP:s ( 52, 72 ) select the modulation scheme according to modulation information ( 49, 69 ). In such a way, a switching between different modulation schemes can be performed even on a time-slot basis. For non-constant-envelope modulation, the modulated signal is separated into two component signals. Each TRU ( 50, 70 ) takes care of the amplification of one component. A phase compensation of at least one of the TRU:s ( 50, 70 ) is performed in order to correct for different paths of phases of the power amplifiers ( 64, 84 ). The non-constant envelope modulated signal can also be a multi-carrier signal, e.g. of two or more constant-envelope signals. Also a TCC (Transmitter Coherent Combining) operation is achievable.

This application is the US national phase of international applicationPCT/SE03/00459 filed in English 19 Mar. 2003, which designated the US.PCT/SE02/00459 claims priority of SE Application No. 0201070-0, filed 2Apr. 2002. The entire contents of these applications are incorporatedherein by reference.

TECHNICAL FIELD

The technology of this disclosure relates in general to wirelesscommunication and in particular to wireless communication transmittersystems.

BACKGROUND

In conventional GSM (Global System for Mobile Communication), amodulation scheme according to GMSK (Gaussian Minimum-Shift Keying) isimplemented. GMSK is a constant envelope modulation scheme, where aphase shift is differentially dependent on the bit sequence. The GMSKmodulation has been chosen as a compromise between fairly high spectrumefficiency and reasonable demodulation complexity.

EDGE (Enhanced Data for Global Evolution) is a high-speed mobile datastandard, intended to enable second-generation GSM and TDMA (TimeDivision Multiple Access) networks to transmit data up to 384 kbps. EDGEprovides the speed enhancement by changing the type of modulation usedand making better use of the carrier currently used. It enables agreater data transmission speed to be achieved in good conditions, inparticular near the base stations by implementing 8PSK(Eight-Phase-Shift Keying) modulation. The 8PSK modulation scheme is ahigh transmission modulation based on phase shift coding. The modulationis of a non-constant envelope type. EDGE can co-exist with the existingGSM traffic, switching to EDGE mode when appropriate.

When upgrading a base station to handle EDGE, the transmitter system hasto be modified. A transmitter used for standard GSM purposes is designedfor supporting GMSK, which means that the power amplifier that are usedtypically are more or less non-linear. When implementing 8PSK, theenvelope may vary in a pre-defined way over time, and non-linearamplification can not be accepted. Thus, in a general case, a newparallel transmitter arrangement has to be provided. Since thetransmitter devices are costly, parallel transmitter arrangements, whichare only used one at a time, means a poor utilization of installedequipment. Furthermore, highly linear power amplifier elements orarrangements are very expensive and there is a request to avoidsolutions using such elements.

In “Increasing the talk-time of mobile radios with efficient lineartransmitter architectures” by S. Mann, M. Beach, P. Warr and J.McGeehan, Electronics & Communication Engineering Journal, April 2001,Vol. 13, No. 2, pp. 65-76, the relationship between linearizing methodsfor power amplification in radio transmitters and efficiency isdiscussed. LINC (LInear Nonlinear Component), known in prior art e.g. byU.S. Pat. No. 5,990,734, is one of the investigated schemes, where onenon-constant envelope signal is divided into two constant envelopesignals, which subsequently can be amplified by non-linear amplifiers.However, since such a method requires two non-linear amplifiers, this isnot a particularly efficient approach for systems also handling constantenvelope signals.

SUMMARY

An object of one or more non-limiting embodiments is to provide forusing one and the same transmitter system for constant-envelope as wellas non-constant envelope modulation schemes. Another object is toprovide a transmitter system for non-constant envelope modulationschemes based on non-linear power amplifier elements. A further objectis to provide the possibility for fast switching between differentmodulation schemes.

The above objects are achieved by one or more non-limiting exampleembodiments. In general, a double TRU (Transceiver Unit) is used. Theoutput signals from the power amplifiers are combined to one commonoutput signal provided to an antenna arrangement. A DSP (Digital SignalProcessor) of each TRU comprises means for a constant-envelopemodulation scheme and a non-constant envelope scheme. The DSP:s selectthe modulation scheme according to modulation information providedtogether with the input digital signal. In such a way, a switchingbetween different modulation schemes can be performed even on atime-slot basis.

In case of a non-constant-envelope modulation, the DSP divides themodulated signal into two component signals. Each TRU takes care of theamplification of one component, and the components are eventuallycombined before being provided to the antenna arrangement. A phase LOcompensation of at least one of the TRU:s is performed in order tocorrect for different paths or phase positions of the power amplifiers.The non-constant envelope modulated signal can also be a multi-carriersignal, e.g. of two or more constant-envelope signals.

For normal constant-envelope modulation, the two TRU:s are operatingindependently of each other, and the two output signals are combined toa double-carrier signal.

The arrangement can also be operated according to TCC (TransmitterCoherent Combining) of constant-envelope modulated signals, where bothTRU:s are provided with the same digital signal. The two amplifiedoutput signals are combined to create an output signal of double theamplitude. Also here, phase compensation is desirable.

The phase compensation is preferably determined by monitoring the outputpower or monitoring the power in the load of the hybrid and comparingwith expected output power. In one non-limiting example embodiment, acalibration of the phase compensation is performed during TCC bursts,and utilized during non-constant envelope modulation. Other non-limitingexamples embodiments utilize constant amplitude portions of non-constantenvelope time slots for performing phase compensation calibration. Onemay then make use also of power measurements of the output signals fromeach power amplifier. The phase compensation calibration can also beperformed during well-characterized training sequences within the timeslots.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, maybest be understood by making reference to the following descriptiontaken together with the accompanying drawings, in which:

FIG. 1 is a vector diagram illustrating a constant envelope signal;

FIG. 2 is a vector diagram illustrating a non-constant envelope signal;

FIG. 3 is a vector diagram illustrating principles of separating anarbitrary signal into two constant envelope signals;

FIG. 4 is a vector diagram illustrating the principles of transmittercoherent combining;

FIG. 5 is a vector diagram illustrating the effect of phase errors intransmitter coherent combining;

FIG. 6 is a block diagram illustrating a non-limiting example embodimentof a double transmitter unit according to the present invention;

FIG. 7 is a flow diagram illustrating a non-limiting example method forproviding two constant envelope modulated signals according to thepresent invention;

FIG. 8 is a flow diagram illustrating a non-limiting example method forproviding a non-constant envelope modulated signal according to thepresent invention;

FIG. 9 is a flow diagram illustrating a non-limiting example methodproviding transmitter coherent combining according to the presentinvention;

FIG. 10 is block diagram illustrating another non-limiting exampleembodiment of a double transmitter unit according to the presentinvention;

FIG. 11 is a flow diagram illustrating another non-limiting examplemethod for providing a non-constant envelope modulated signal accordingto the present invention;

FIG. 12 is a flow diagram illustrating another non-limiting examplemethod providing transmitter coherent combining according to the presentinvention;

FIG. 13 is a diagram illustrating a power versus time mask for an 8PSKmodulated normal burst;

FIG. 14 is a block diagram of a part of yet another non-limiting exampleembodiment of a double transmitter unit according to the presentinvention;

FIG. 15 illustrates a time slot used in GMSK or 8PSK modulation;

FIG. 16 is a block diagram illustrating an alternative phase shiftersolution applicable or more non-limiting examples the present invention;

FIG. 17 is a diagram illustrating the principles of the phase shifter ofFIG. 16;

FIG. 18 is a block diagram illustrating a part supporting frequencyhopping of a non-limiting example embodiment of the present invention;

FIG. 19 is an illustration of a storage of phase shifts usable togetherwith one or more non-limiting examples the present invention; and

FIG. 20 is a block diagram of one transmitter unit supporting doublecarrier signals of a non-limiting example embodiment of a doubletransmitter unit according to the present invention.

DETAILED DESCRIPTION

A signal modulated according to the GMSK modulation scheme can bevisualized in the complex signal plane as illustrated in FIG. 1. Thecoordinate system is here supposed to rotate synchronously with the basefrequency of the carrier, and only the phase differences will thereforeappear in the diagram. A modulated signal is thereby represented by avector 10. In GMSK, the phase shift is adjusted according to threesuccessive bits in the digital input signal. Generally speaking, thephase is smoothly shifted π/2 if the three successive digits are thesame. This means that the vector 10 shifts counterclockwise according toarrow 11 if the successive digits are the same and clockwise accordingto arrow 12 if they are different. All the time, the so vector 10maintain its magnitude, i.e. the end of the vector 10 always travels ona circle 13 in the I-Q-space. The modulation scheme is therefore said tobe a constant-envelope scheme. It is relatively easy to amplify aconstant-envelope signal, since also non-linear power amplifiers may beused. Since the signal always has one and the same envelope, the gain isalways the same, regardless of the linearity of the amplifier. Simplerpower amplifier solutions can thereby be used.

A signal modulated according to the 8PSK modulation scheme can be alsobe visualized in the complex signal plane as illustrated in FIG. 2. Thecoordinate system is also here supposed to rotate synchronously with thebase frequency of the carrier, and only the phase differences willtherefore appear in the diagram. A modulated signal is therebyrepresented by a vector 20. In 8PSK, the phase shift is a coding of atriplet of binary digits. A certain phase shift corresponding to aspecific set of digits, as indicated by points 21 in the figure.Depending on the scheme, the assignment may vary, and there might alsobe an offset phase shift present, which removes the points from theaxes. However, the example in FIG. 2 illustrates well the principle.When changing from one triplet to the next, the vector moves from onepoint 21 to another and passes thereby through the interior of thecircle 13. The modulation scheme is therefore said to be anon-constant-envelope scheme. When amplifying an 8PSK signal, theamplifier arrangement has to have relatively linear characteristics,since the signal will change its magnitude. Highly linear amplifiers areexpensive and simpler solutions are desired.

One possible approach to provide a linear amplification is to decomposeor separate the signal into two component signals, amplify thesecomponent signals and combine the amplified component signals again. Ifone keeps the component magnitudes constant, even non-linear amplifierscan be used. This principle, LINC (LInear Nonlinear Component)amplifier, is known in prior art, e.g. by U.S. Pat. No. 5,990,734. Onethereby trades one linear amplifier for two non-linear ones, plus aseparator and combiner. FIG. 3 illustrates the principles. A signal tobe amplified is represented by a vector 30. The vector 30 has a varyingsize S and phase shift φ. The vector 30 is separated into two componentvectors 31, 32. In a first embodiment, the amplitude A of the componentsis the same and follows the circle 13. This amplitude has to be at leasthalf of the maximum amplitude of the vector 30. Also the phasedifference a to the vector 30 is the same, however, directed in oppositedirections. According to basic geometrical considerations, the componentphase shifts θ₁ and θ₂ are specified by:θ₁=φ−arccos(S/2A)θ₂=φ+arccos(S/2A).

As anyone skilled in the art understand, it is by this possible toexpress any arbitrary vector of length ≦2A by two component vectors oflength A. The component signals can then be amplified according toprinciples of amplifying constant-envelope signals.

In a more general case, the amplitudes of the components may bedifferent and may also vary depending on the size and phase of thevector 30. Such applications will be discussed more in detail below.

A special case of combining two component signals into one final outputsignal is in case two component signals always with the same phase arecombined. This can be used in cases where a high amplification isdesired, and where it is difficult to achieve by only one amplifier. Asshown in FIG. 4, two components 41, 42 (seen as one vector in thefigure) of the same phase can then be amplified separately and combinedinto an output signal 43. This is the basic idea of TCC (TransmitterCoherent Combining).

A practical problem in combining two separate signals into one outputsignal is that the paths through the amplifiers typically involves somepath difference or that the devices are locked in different phasepositions, which will be noticed as a small phase shift between the twocomponent signals. Such a situation is illustrated in FIG. 5. A signal30 is separated into two components 31, 32. During amplification, thefirst one of the components is shifted Δθ compared to the second one.This phase shifted components is illustrated by a broken arrow 34. Theactual composed output signal 33 will then be changed both in phase andamplitude. One solution is to measure the phase shift differencecarefully and compensate for it by introducing phase altering means inone of the paths. However, such a phase shift may also be slowly varyingwith time, and in such cases, an adaptive phase compensating arrangementhas to be introduced. A preferred embodiment of such an arrangement willbe described farther below.

A non-limiting example embodiment of a double transmitter unitarrangement 45 is illustrated in FIG. 6. A first modulation unit 50 hasan input 51 for receiving a digital signal to be transmitted. The input51 is connected to a DSP (digital signal processor) 52. The DSP 52comprises modulation means; a 8PSK modulator 53 and a GMSK modulator 54.The DSP 52 also comprises a control input 49 for receiving modulationinformation, and a selector 55. The selector 55 selects one of themodulators 53, 54 according to the modulation information received bythe control input 49. The digital signal received by the input 51 isthereby provided to one of the modulators 53, 54. The different means inthe DSP 52 can be implemented as software.

The GMSK modulator 54 modulates the input digital signal according tothe GMSK scheme. The modulated signal is in this embodiment provided ina real I and an imaginary part Q at two outputs, connected to ananalogue signal generator 56. In this embodiment, the analogue signalgenerator 56 comprises a quadrature modulator 57. The analogue signalgenerator 56 also comprises two DAC's (Digital-to-Analogue Converters)58, 59 converting the I and Q signals, respectively, into analoguevoltages. The analogue voltages are modulated in a mixer 60 with thecarrier frequency, provided by a frequency generator 61, and combined. Aphase shifter 62 shifts the frequency signal to the Q component by 90degrees. The output from the analogue signal generator 56 is thus ananalogue voltage signal being modulated, in this ease according to theGMSK scheme.

A phase shifter 63 is in this embodiment connected between themodulators 53, 54 and the quadrature modulator 57. The function of thisunit will be described more in detail further below. The analogue signalfrom the analogue signal generator 56 is provided to a power amplifier64 for amplification. In the present embodiment, the power amplifier 64is a non-linear amplifier. The amplified signal from the output of thepower amplifier 64 is provided to an input of a hybrid combiner device90.

The output of the 8PSK modulator 53 is provided to a separator 65. Theseparator separates the signal provided from the 8PSK modulator into twocomponents, whereby the input signal is the vector sum of the twocomponents. A first one of the components is provided to the input ofthe analogue signal generator 56, in the form of an I and a Q signal. Inthe present embodiment, the second component is terminated. The firstcomponent is processed in the analogue signal generator 56 in the samemanner as described above.

A second modulator unit 70 is very similar to the first modulation unit50. It has an input 71 for a digital signal and a control input 69. ADSP 72 comprises analogously a 8PSK modulator 73 and a GMSK modulator74, and a selector 75 selecting which of the modulator that is going tobe used.

The GMSK modulator 74 is similarly connected to an analogue signalgenerator 76, having a quadrature modulator 77. However, no phaseshifter is present. The quadrature modulator 77 is of the same structureas the one in the first modulation unit 50.

The analogue signal from the analogue signal generator 76 is provided toa power amplifier 84 for amplification. In the present embodiment, thepower amplifier 84 is a non-linear amplifier of the same type as thepower amplifier 64. The amplified signal from the output of the poweramplifier 84 is provided to a second input of the hybrid combiner device90.

The output of the 8PSK modulator 73 is provided to a separator 85,having the same function as the separator 65. The separator 85 separatesthe signal provided from the 8PSK modulator into two components, wherebythe input signal is the vector sum of the two components. In the presentembodiment, the first component is terminated. The second component isinstead provided to the input of the analogue signal generator 76, inthe form of an I and a Q signal. The second component is processed inthe analogue signal generator 76 in the same manner as described above.

The hybrid combiner device 90 combines the two signals provided by theoutputs of the two power amplifiers 64, 84 into a transmitter signal,that is provided to a transmitted device 91. The input power supplied bythe power amplifiers 64, 84 is at least to a part provided as atransmitter signal power. However, any remaining power will bedissipated by a hybrid load 92.

In the present embodiment, the power dissipated over the hybrid load 92is measured by a power meter 93. The output of the power meter 93 isconnected to the phase shifter 63 via an ADC (Analogue-to-DigitalConverter). The value of the hybrid load power is provided to a phasecontroller 94, which calculates any phase shift between the amplifiedsignals provided to the hybrid. The phase shifter 63 further comprises acomplex multiplier 95, providing a digital phase shift angle e^(i) ^(Δθ)to the I and Q signals respectively. This phase shift is thus in acomplex manner incorporated in the I and Q signals that are enteringinto the analogue signal generator 56.

By this double transmitter unit arrangement 45, a number of differentmodulation techniques can be employed. By accompanying the digitalsignals with associated modulation information, the switching betweendifferent modulation schemes can be performed very swiftly, even on atime slot basis. Such an arrangement thus allows the transmitterarrangement 45 to allow for mixing e.g. GMSK bursts with 8PSK traffic ona time slot basis.

Some examples of different operation modes of the double transmitterunit 45 are given here below. Assume normal GMSK traffic. The doubletransmitter unit 45 then operates as two independent transmitter paths,having one carrier each. A digital signal of a first carrier is providedto the first modulation unit 50, while a digital signal of a secondcarrier is provided to the second modulation unit 70. The modulationinformation instructs both DSP's 52, 72 to select a GMSK modulation. Thetwo carrier signals are combined in the hybrid combiner 90 into a commonsignal, provided to the transmitter. The phase shifter arrangement is inthis case not used.

In case a GMSK signal with a high output power is desired, a TCCarrangement can be achieved. In such a case, the same digital signal isprovided to both modulation units 50, 70 together with a request forGMSK modulation. Both transmitter units are processing the same signaland the combined signal at the hybrid combiner 90 output is ideally ofdouble the output power. In comparison with combining two differentcarriers, the TCC carrier is provided with a power four times higher.This is due to the fact that half the power dissipates in the load whencombining two different carriers, while coherent combining removes allpower from the load. However, as discussed above, any phase shiftscaused by path differences in the two branches may deteriorate the totalsignal. In this TCC arrangement, the phase shifter 63 comes into use. Inthis embodiment, the power over the hybrid load 92 is measured. If theamplifier branches are perfectly aligned in phase, all power will bedistributed to the transmitter device 91, which means that no power willbe dissipated through the hybrid load. By adjusting the phase of thesignal in one of the paths, the hybrid load power can be minimized,which indicates an alignment in phase of the two components.

A third operational mode is when a 8PSK signal is to be transmitted.Also in this case, both inputs 51, 71 are provided with the same digitalsignal. This signal will be modulated according to the 8PSK scheme sincethe selectors 55, 75 selects the 8PSK modulator 53, 73. The separator 65in the DSP 52 of the first modulation unit 50 provides a first componentsignal to the analogue signal generator 56. The separator 85 in the DSP72 of the second modulation unit 70 provides instead a second componentsignal to the analogue signal generator 76. The vector sum of these twocomponents equals the original 8PSK-modulated signal. Each of thecomponents are amplified in a separate power amplifier 64, 84, andcombined in the hybrid combined device 90 to form an amplified versionof the original signal. The double transmitter unit arrangement 45 thushere operates at least partly in accordance with the LINC concept,providing one 8PSK carrier signal. Also here, phase shifts between thepaths may appear. Different approaches for solving this problems arediscussed further below.

In FIG. 7-9, the above operations are illustrated as flow diagrams.First, in FIG. 7, a non-limiting example method of providing two GMSKsignals on one carrier each is illustrated. The procedure starts in act100. In act 101, a first digital signal is provided to a firsttransmitter unit. This first digital signal is intended to betransmitted on a first carrier. In act 102, a second digital signal isprovided to a second transmitter unit. This second digital signal isintended to be transmitted on a second carrier. In act 103,constant-envelope modulation information is provided to the firsttransmitter unit. In act 104, constant-envelope modulation informationis provided to the second transmitter unit. A. constant-envelopemodulation scheme is selected and applied in the first transmitter unitaccording to the modulation information in act 105, and aconstant-envelope modulation scheme is selected and applied in thesecond transmitter unit according to the modulation information in act106. A first analogue signal corresponding to the first digital signalmodulated according to the information is generated in act 110. A secondanalogue signal corresponding to the second digital signal modulatedaccording to the information is generated in act 111. In act 112, thefirst analogue signal is amplified and in act 113, the second analoguesignal is amplified. In act 114, the two amplified signals are combinedto a two-carrier output signal to be transmitted. The procedure is endedin act 115.

FIG. 8 illustrates an example non-limiting method of providing a signalmodulated according to a 8PSK modulation according to the presentinvention is illustrated. The procedure starts in act 120. In act 121, adigital signal is provided to a first transmitter unit and the samedigital signal is also provided to a second transmitter unit. In act 123non-constant-envelope modulation information is provided to the firsttransmitter unit and to the second transmitter unit. Anon-constant-envelope modulation scheme is selected and applied in thefirst transmitter unit according to the modulation information in act125, and a non-constant-envelope modulation scheme is selected andapplied in the second transmitter unit according to the modulationinformation in act 126. In act 127, the modulated signal in the firsttransmitter is separated into a first and a second component. In act 128the modulated signal in the second transmitter is separated into thesame first and second components. The first component is phase shiftedin act 129 to compensate for differences in phase characteristicsbetween the paths through amplifier stages of the first and secondtransmitter unit, respectively. A first analogue signal corresponding tothe first phase-shifted component is generated in act 130. A secondanalogue signal corresponding to the second component is generated inact 131. In act 132, the first analogue signal is amplified and in act133, the second analogue signal is amplified. In act 134, the twoamplified signals are combined to a single-carrier output signal to betransmitted. The procedure is ended in act 135.

FIG. 9 illustrates the case of a TCC operation. The procedure starts inact 140. In act 141, a digital signal is provided to a first transmitterunit and the same digital signal is provided also to a secondtransmitter unit. This digital signal is intended to be transmitted witha double intensity. In act 143, constant-envelope modulation informationis provided to the first transmitter unit and to the second transmitterunit. A constant-envelope modulation scheme is selected and applied inthe first transmitter unit according to the modulation information inact 145, and a constant-envelope modulation scheme is selected andapplied in the second transmitter unit according to the modulationinformation in act 146. The first modulated signal is phase shifted inact 149 to compensate for differences in phase characteristics betweenthe paths through amplifier stages of the first and second transmitterunit, respectively. A first analogue signal corresponding to the firstdigital signal modulated according to the information and phase-shiftedis generated in act 150. A second analogue signal corresponding to thesecond digital signal modulated according to the information isgenerated in act 151. In act 152, the first analogue signal is amplifiedand in act 153, the second analogue signal is amplified. In act 154, thetwo amplified signals are combined to double the amplitude of asingle-carrier output signal to be transmitted. The procedure is endedin act 155.

The three flow diagrams exhibit large resemblances. The changes in thedifferent acts are of such a character, that it can be changed by e.g.software as a response on e.g. the modulation information given. Suchinformation can be provided on a time-slot basis, i.e. the requestedmodulation can be changed from one time-slot to the next. This impliesthat also the different operational modes are interchangeable from onetime-slot to the next.

Some modifications of the above embodiments are also interesting to bedisclosed. In FIG. 10, another embodiment of a double transmitterarrangement according to the present invention is illustrated. Mostparts are identical to the ones in the first illustrated embodiment, andwill not be discussed again. However, some clear differences arepresent. First of all, it can be noted that in the previous embodiment,identical modulation and separation operations are performed in parallelin the first and second modulation units 50, 70. This can be avoided bythe present design, in which the second modulation unit 70 does notexplicitly have any separator. Instead, a connection 66 connects theoutput for the second component of the separator 65 in the firstmodulation unit 50 with the input of the analogue signal generator 76 ofthe second modulation unit 70. In this manner, the second modulationunit 70 can be made somewhat simpler and the computational effort duringthe operation is concentrated to the first modulation unit 50. Aconnection 67 also connects the output of the GMSK modulator of thefirst modulation unit 50 with the input of the analogue signal generator76 of the second modulation unit 70. This enables the correspondingsimplification to be performed for the TCC operation.

FIG. 11 illustrates a flow diagram corresponding to the 8PSK operationwith the embodiment of FIG. 10. Since the steps present are identicalwith some of the steps of the procedure of FIG. 8 they are not discussedagain. Basically, the steps 126 and 128 are omitted and the steps 121and 123 are changed into steps 122 and 124 respectively, in which onlythe first transmitter unit is involved. The second component used instep 131 is in this embodiment, however, provided from the firstmodulation unit.

FIG. 12 illustrates a flow diagram corresponding to the TCC operationwith the embodiment of FIG. 10. Since the steps present are identicalwith some of the steps of the procedure of FIG. 9 they are not discussedagain. Basically, the step 146 is omitted and the steps 141 and 143 arechanged into steps 142 and 144 respectively, in which only the firsttransmitter unit is involved. The second modulated signal used in step151 is in this embodiment, however, provided from the first modulationunit.

Now returning to FIG. 6. In this embodiment, the phase shifting of thesignal provided to the first power amplifier 64 was based on ameasurement of the power of the hybrid combiner load 92. Since there isa complementary relationship between the power to the transmitter device91 and the load 92, either power can be measured and the other can becalculated. Measuring the load power is a relatively easy task, but ofcourse, a direct measuring of the power supplied to the transmitterdevice is possible. The evaluation performed in the phase shifter 63 hasof course to be changed accordingly.

The phase shifting during TCC operation is relatively straight-forward.The power dissipated in the load 92 is minimised, and the two signalsare thereby phase-synchronized. However, in the case of 8PSK operation,the possible manners of performing the phase-shifting are less obvious.In a system, where the flexibility of the present invention is fullyused, the character of the transmitted signals varies. If TCC operationappears occasionally, the phase-shifting can be calibrated during suchTCC time slots. The values of the optimum phase shift can then bestored, e.g. in the phase shifter 63, to be used e.g. during 8PSKoperation.

The situation may, however, be somewhat more complex if the arrangementis designed also for frequency hopping. In FIG. 18, a part of a doubletransmitter arrangement is illustrated. The analogue signal generator 56of the first modulation unit 50 is illustrated to have access to twodifferent frequency generators 61A and 61B. A switch 68 connects onefrequency generator at a time to the quadrature modulator 57. In themeantime, the other frequency generator is controlled to be tuned to thenext frequency to use. When the frequency change is to be carriedthrough, the switch 68 selects the other frequency generator. Eachfrequency used may influence the amplifier equipment to give differentphase shifts. This means that the phase shift applied to the signal tobe amplified in e.g. TCC or 8PSK mode has to be calibrated at thatparticular frequency. If the phase shifts are calibrated during ICC modeand stored to be used in 8PSK mode, there has to be one phase shiftvalue for each frequency used by the arrangement. Also, the twofrequency generators 61A and 61B may give rise to different phaseshifts, whereby one calibrated phase shift for each combination offrequency generator and frequency is needed. A signal can be sent fromthe frequency generators 61A, 61B to the phase shifter 63 by aconnection 86, for instructing the phase shifter which phase shift toapply.

A storage 87 of phase shifts, e.g. comprised in the phase shifter 63 canbe configured as illustrated by FIG. 19. The storage 87 is here designedas a look-up table, with two input variables, identity of frequencygenerator used and the frequency of that frequency generator.

There are also alternative ways of obtaining calibrated phase shifts forthe 8PSK operation. These are necessary in cases there are no or veryfew TCC time slots. If only one power measure is available, e.g. thepower dissipated in the load 92 (FIG. 8), there has to be some inherentknowledge of the expected power to the transmitter device. In FIG. 13, aPVT (Power Versus Time) mask for 8PSK modulated normal burst is shown.The PVT mask defines the envelope range in which the 8PSK signal isallowed to vary. At a short time period before 200 and after 202 themain signal period 204, the maximum and minimum power curves areseparated by only 2.4 dB. This implies that without any knowledge of theactual system, the actual power of the signal is known with an accuracyof at least 2.4 dB. However, in most cases, design considerations areknown and the accuracy of the power is generally much higher, in atypical case 0.3-0.5 dB. By performing an output power measurementduring at least one of these periods a calibration of the phase shiftcan be achieved, even though the main signal is of a non-constantenvelope type. The power level in this period has a known relationshipto the average power over the entire burst. If the phase error of theamplifier varies with output power, the maximum value for the envelopewill be in phase, while the phase shift at dips in the envelope maydiffer. The phase shift monitoring is thus performed during a period oftransmission of a constant amplitude signal within a non-constantenvelope signal.

If any correction of the phase shift is needed, a correction of thephase shift added to the first signal is preferably performed when nouseful signal is transmitted from the transceiver device, e.g. during aguard period between two time slots. Since the guard time is long enoughto perform all setting procedures for the new phase shift, this willensure that the signal transmitted during the following time slot doesnot have any defects caused by a phase-shifting in progress.

In the above case, an a-priori knowledge about the expected amplitude ofthe transmitted signal is required. However, in more generaltransmission situations, such knowledge is not always available. In FIG.14, another embodiment of a power meter 93 is illustrated. Only parts,which are directly involved, are illustrated. In this embodiment, thepower meter 93 is still connected to measure the power over the load 92.However, the power meter 93 now also is supplied with signals from afirst and a second power sensor 96, 97, measuring the output power fromthe power amplifiers 64 and 84, respectively. In this manner, the powermeter can keep track on the power entering the hybrid combiner and thepower exiting from it. A signal corresponding to

$1 - \frac{P_{L}}{\left( {P_{TX1} + P_{TX2}} \right)}$where P_(L) is the power dissipated over the load and P_(TX1) andP_(TX2) are the powers of the amplifier outputs. This quantitycorresponds to the cosine factor between the signals from the poweramplifiers. The phase shifter 63 (FIG. 6) can then according to thisadjust any phase shift, if necessary. Such an arrangement may be veryuseful, e.g. if downlink power control is applied.

By measuring also the power of the components, it becomes possible toperform calibration of the phase shift also during periods in whichnon-constant envelope signals are transmitted. However, performing itduring an arbitrary signal section induce a lot of problems. Onesolution is, however, to use signal sections of a-priori known digitalcontent. When transmitting a time slot of data using e.g. GMSKmodulation or 8PSK modulation, a section of “training symbols” isincluded in the data. This is schematically illustrated in FIG. 15.These training symbols are well-known and an expected output signal caneasily be calculated. By monitoring power values according to FIG. 14,during the transmission of such training symbols, an actual outputsignal can be compared with the expected one, and a phase difference canbe detected and used for calibration purposes.

Above, one embodiment of shifting the phase of a signal is illustrated.However, anyone skilled in the art understands that also otherphase-shifting devices and methods can be employed. When operating inthe TCC mode, one attractive alternative arises. FIG. 16 illustratessome selected parts of a transmitter arrangement having an alternativephase-shifting arrangement. The power meter 93 is as before connected toa phase shifter 63. However, in this embodiment, the phase shifter 63 isdirectly connected to the GMSK modulation means in the DSP 52. The phaseshifter 63 evaluates the power signals from the power meter 93 andprovides a requested phase shift AO to the GMSK modulation means 54. TheGMSK modulation means 54 uses typically a tabulated state machine 98operating according to a transfer function between the phase shiftinduced by the digital signal and time. A graph of such a function isillustrated in FIG. 17. The transfer function is draw with a full lineand denoted by 210. By simply adding the phase shift Δθ provided by thephase shifter 63 to the value achieved from the transfer function, theentire signal will be provided with an additional phase shift. Thephase-shift compensated transfer function will then look like the brokenline 212.

In the above embodiments, the DSP's 52, 72 have comprised oneconstant-envelope modulation means and one non-constant envelopemodulation means, in the form of a GMSK modulator and an 8PSK modulator.The DSP's may also comprise different types and different number ofmodulators. Other types of phase shift keying, such as 4PSK, areexamples of possible other non-constant envelope modulators.

Another interesting non-constant envelope modulator that can be used inthe present invention is a modulator for combined carrier signals. Oneembodiment of such a multi-carrier modulator is illustrated in FIG. 20.Here, two carriers of a GMSK modulation are combined, but it is alsopossible to combine carriers of other modulation schemes, e.g. 8PSK.Also, it is possible to combine carriers having different modulationschemes, e.g. one GMSK and one 8PSK carrier. Moreover, the basic ideasof this carrier combining can be generalized into more than twocarriers. However, in such cases, bandwidth restrictions may set apractical limit.

The DSP 52 comprises a carrier combiner modulation means 220, in thepresent embodiment in turn comprising two GMSK modulators 54A, 54B. Oneof the outputs of the selector 55 is connected to the first GMSKmodulator 54A. The first GMSK modulator means 54A is thus provided withthe digital signal provided by the input 51, which represents the signalintended for the first carrier. An additional digital signal input 228is provided to the second GMSK modulator 54B, whereby this modulator isprovided with a digital signal, which represents the signal intended forthe second carrier. An additional information input 222 is provided,which carries data defining a frequency difference between the twocarriers, or in the present embodiment half this frequency difference.The digital signals are GMSK-modulated separately into digital I and Qrepresentations. The I and Q representation from the first GMSKmodulator 54A is then modulated in a pre-modulator 225 with a signalhaving half the difference frequency provided by input 222, but with anopposite phase direction, i.e. in practice minus half the differencefrequency. The I and Q representation from the second GMSK modulator 54Bis similarly modulated in a pre-modulator 226 with a signal having halfthe difference frequency provided by input 222. The digital I and Qsignals are finally added in a summing means 224, providing a signalrepresenting two digital signals on one carrier each, pre-modulated tofrequencies of ±Δf/2. The up-conversion of the frequency, taking placelater in the chain, the frequency is selected to be the mean frequencyof the two carriers.

The digital signals resulting directly from the GMSK modulators 54A, 54Bare constant envelope signals. However, after the pre-modulation by thedifference frequency, they exhibit a non-constant envelope behavior. Thecomplex sum of these two signals is also of a non-constant envelopecharacter. In analogy with the 8PSK case described above, it is possibleto separate this sum signal into two components 31, 32 with constantenvelopes (cf. FIG. 4). The process then continues in analogy with the8PSK case described further above.

By using this scheme, any arbitrary combination of modulation schemes inany number of carriers can be combined and processed as a non-constantenvelope signal. Since the choice of modulation schemes furthermore canbe performed on a time slot basis, this opens up for a very highflexibility in the use of the transceiver unit arrangement according tothe present invention. However, there are also some drawbacks present.First of all, since the frequency difference between the carriers ismodulated into the signal even before the separation into components,the bandwidth of the signals that has to be treated throughout thetransceiver unit path is increased. The increase in bandwidthcorresponds approximately to the frequency difference. This puts veryhigh demands on the components in the transceiver unit, in particular onthe DAC's. There are, however, already today DAC components that wouldbe able to handle at least neighboring frequencies. Using more than twocarriers will of course make the bandwidth requirements even larger.

Another problem is that, if using more than two carriers, the outputpower per carrier will decrease. Since the total power is restricted bythe sum of the power of each individual transceiver unit, this maximumpower can not be exceeded. When having three or more carriers, the sumsignal 15 has to be scaled down in order to assure that it can beseparated into components, i.e. it has to be kept within double thecomponent amplitude. In order to be absolutely sure that every possiblecombination will be covered, the output power of each carrier will bereduced by a factor n/2, where n is the number of carriers.

The principle of separating a non-constant envelope signal into constantenvelope components opens up for a very flexible use of the transceiverunits. However, this principle is not very power efficient when handlingsignals of low amplitude. Even if the total signal has a low amplitude,the components have high amplitudes, which means that a large portion ofthe power will be wasted when re-combining the components in the hybridcombiner. A large power will dissipate through the load.

Also, when the total signal has a low amplitude, small changes in thesignal may cause very large phase changes of the components. Thebandwidth necessary to process the components will therefore be largerwhen the total signal has a low amplitude.

A way to reduce the problems described above is to renounce the demandof keeping the component amplitude constant. By letting the componentamplitude decrease when the total signal amplitude becomes small, someadvantages are achieved. The required bandwidth will decrease and thetotal power efficiency will increase. However, such component amplitudevariations should be kept within certain limits.

Another aspect to consider when deciding the reduction of the componentamplitudes is the efficiency of the power amplifiers. Most poweramplifiers exhibit the highest efficiency at the highest output values.A too large reduction in component amplitude will indeed result inhigher efficiency in the combiner stage, but may reduce the efficiencyin the power amplifier even more. The component amplitude reduction isthus preferably performed to optimize the allover efficiency.

As described in the above embodiments, there are a number of interestingadvantages arising by using one or more non-limiting examples of thepresent invention. One of the main advantages is the high flexibility inusing the arrangement. A user may easily, even on a time slot basis,change between different transmitting configurations. It is thuspossible to change e.g. between high capacity and high output power,depending on the actual need. No re-calibrations have to be performedand the changes typically involve solely software changes.

It will be understood by those skilled in the art that variousmodifications and changes may be made to the present invention withoutdeparture from the scope thereof, which is defined by the appendedclaims.

REFERENCES

-   S. Mann, M. Beach, P. Warr and J. McGeehan, “Increasing the    talk-time of mobile radios with efficient linear transmitter    architectures”, Electronics & Communication Engineering Journal,    April 2001, Vol. 13, No. 2, pp. 65-76.-   U.S. Pat. No. 5,990,734.

1. A transmitter arrangement, comprising: a first modulation unit havinga first digital signal processor and a first analogue signal generator;said first digital signal processor having a first digital signal input;a first power amplifier, connected to an output of said first analoguesignal generator; a second modulation unit having a second digitalsignal processor and a second analogue signal generator; said seconddigital signal processor having a second digital signal input; a secondpower amplifier, connected to an output of said second analogue signalgenerator; a combiner device connected to outputs of said first and saidsecond power amplifiers; a transmitter device connected to an output ofsaid combiner device; a first power monitor sensing a total power tosaid transmitter device or a quantity directly related thereto; and aphase-shifter connected to said first power monitor, arranged forcausing a phase shift of an analogue signal generated by said firstanalogue signal generator in response to said sensed total power,wherein said first digital signal processor further comprises: at leastone first non-constant envelope modulation means; a first signalcomponent separator connected to an output of said at least one firstnon-constant envelope modulation means; a first output of said firstsignal component separator being connectable to said first analoguesignal generator; first means for receiving modulation instructions; atleast one first constant envelope modulation means connectable to saidfirst analogue signal generator; and first modulation selecting meansfor connecting a modulation means to said first digital signal input inresponse to received modulation instructions, said first modulationselecting means being operable on a time slot basis, and wherein saidtransmitter arrangement further comprises means for providing said firstand said second digital signal inputs with a same digital signal, andsaid first and said second means for receiving instructions with thesame instructions of a constant envelope modulation, allowingtransmitter coherent combining.
 2. The transmitter arrangement accordingto claim 1, wherein said second digital signal processor furthercomprises: at least one second non-constant envelope modulation means ofthe same type as said at least one first non-constant envelopemodulation means; and a second signal component separator connected toan output of said at least one second non-constant envelope modulationmeans, wherein an output of said second signal component separator beingconnectable to said second analogue signal generator, and a sum of asignal of said first output of said first signal component separator anda signal of said output of said second signal component separator beingequal to a signal of said output of said at least one first non-constantenvelope modulation means.
 3. The transmitter arrangement according toclaim 1, wherein a second output of said first signal componentseparator being connectable to said second analogue signal generator. 4.The transmitter arrangement according to claim 1, wherein said seconddigital signal processor further comprises: second means for receivingmodulation instructions; at least one second constant envelopemodulation means connectable to said second analogue signal generator;and second modulation selecting means for connecting a modulation meansto said second digital signal input in response to received modulationinstructions.
 5. The transmitter arrangement according to claim 4,wherein said second modulation selecting means are operable on the timeslot basis.
 6. The transmitter arrangement according to claim 1, whereinsaid first power monitor is a power meter sensing a load of saidcombiner device.
 7. The transmitter arrangement according to claim 1,wherein said phase-shifter comprises means for complex multiplication ofsaid phase shift with a digital signal to be inputted to said analoguesignal generator.
 8. The transmitter arrangement according to claim 1,using GMSK modulation, wherein said phase-shifter comprises means forintroducing a phase offset in said GMSK modulation, generated by using atable driven state machine in said first digital signal processor. 9.The transmitter arrangement according to claim 1, further comprising:second power monitor sensing a power on said output of said first poweramplifier and being connected to said phase-shifter; and third powermonitor sensing a power on said output of said second power amplifierand being connected to said phase-shifter; said phase-shifter beingarranged for causing a phase shift in response to a comparison betweensaid sensed total power and said sensed powers on said outputs of saidfirst and said second power amplifier, respectively.
 10. The transmitterarrangement according to claim 1, wherein that said first and secondnon-constant envelope modulation means are selected from the list of:4-PSK modulation means; 8-PSK modulation means; and means forcombination of at least two carriers.
 11. The transmitter arrangementaccording to claim 4, wherein said first and said second constantenvelope modulation means are GMSK modulation means.
 12. A method forgenerating a transmitter signal in a transmitter arrangement having atleast a first and a second modulation unit arranged in parallel, eachone allowing for at least one non-constant envelope modulation and atleast one constant envelope modulation, said first modulation unithaving a first analogue signal generator, said second modulation unithaving a second analogue signal generator, the method comprising theacts of: providing digital signal to said first and said secondmodulation units; providing modulation information to said first andsecond modulation units; creating a first input signal to said firstanalogue signal generator by performing a constant envelope modulationof a first digital signal provided to said first modulation unit as aresponse of said modulation information being a request for saidconstant envelope modulation, and by performing a non-constant envelopemodulation of said first digital signal and separating a first componentof said non-constant envelope modulated first digital signal as aresponse of said modulation information being a request for saidnon-constant envelope modulation; creating a second input signal to saidsecond analogue signal generator by performing a constant envelopemodulation of a second digital signal provided to said second modulationunit as said response of said modulation information being said requestfor said constant envelope modulation, and by performing saidnon-constant envelope modulation of said first digital signal andseparating a second component of said non-constant envelope modulatedfirst digital signal as said response of said modulation informationbeing said request for said non-constant envelope modulation; generatinga first output signal in said first analogue signal generator accordingto said first input signal; generating a second output signal in saidsecond analogue signal generator according to said second input signal;amplifying said first output signal; amplifying said second outputsignal; combining said first and said second amplified output signals toform an analogue transmitter signal, wherein said providing acts areperformed on a time slot basis, and wherein said modulation informationcomprises said request for said non-constant envelope modulation, andsaid second digital signal is identical with said first digital signal,whereby said act of creating said second input signal to said secondanalogue signal generator is performed on said second signal in saidsecond modulation unit.
 13. The method according to claim 12, whereinsaid modulation information comprises said request for said non-constantenvelope modulation, whereby said act of creating said second inputsignal to said second analogue signal generator is performed on saidfirst signal in said first modulation unit, said method comprising thefurther act of transferring of said second input signal from said firstmodulation unit to said second analogue signal generator.
 14. The methodaccording to claim 13, wherein said non-constant envelope modulation isa 8-PSK modulation.
 15. The method according to claim 13, characterizedin that said non-constant envelope modulation is a multiple-carrier GMSKmodulation, whereby said method comprises the acts of providing a set ofat least two digital signals to both said first and said secondmodulating units, whereby said creating acts comprise the acts ofperforming a GMSK modulation of each digital signal and digitalcombining said modulated signals to form a non-constant envelopemulti-carrier signal, whereby said separating act is performed on saidnon-constant envelope multi-carrier signal.
 16. The method according toclaim 12, wherein said modulation information comprises a request fortransmitter coherent combining of constant envelope modulation signal,and said first digital signal is identical with said second digitalsignal.
 17. The method according to claim 13, comprising the furtheracts of: monitoring a power of said analogue transmitter signal or aquantity directly related thereto; and shifting a phase of said firstoutput signal according to said power.
 18. The method according to claim17, wherein said monitoring act comprises the act of measuring a powerrejected during said combining act, whereby said power of said analoguetransmitter signal is provided as a complementary quantity.
 19. Themethod according to claim 17, wherein said shifting act in turncomprises the act of adjusting an initial offset phase of said first orsaid second modulating in a guard period between two time slots.
 20. Themethod according to claim 17, wherein said shifting act in turncomprises the act of adding a phase shift in connection to thegeneration of the first output signal.
 21. The method according to claim13, wherein said monitoring and said phase shifting is performed when aconstant envelope modulation with transmitter coherent combining isused, whereby said phase shifting is preserved when selecting saidnon-constant envelope modulation.
 22. The method according to claim 13,wherein said monitoring and said phase shifting is performed duringtransmission of a constant amplitude period of said non-constantenvelope signal.
 23. The method according to claim 13, comprising thefurther act of measuring instantaneous power of said first and saidsecond analogue output signals, whereby said shifting is performedaccording to a comparison of said power of said analogue transmittersignal and said power of said first and said second analogue outputsignals.
 24. The method according to claim 23, wherein said shifting inthe case of transmitter coherent combining is performed according to:φ_(shift=cos) ⁻¹(P _(TR)|(P _(TX1) P _(TX2))), where P_(TR) is saidtotal power and P_(TX1) and P_(TX2) are said power of said first andsecond analogue output signals, respectively.
 25. The method accordingto claim 23, wherein said comparison is performed during a period of aknown training sequence in a time slot.
 26. The method according toclaim 12, comprising the further acts of: reducing envelopes of saidfirst and said second signals when said modulated signal has a lowamplitude.
 27. The method according to claim 26, wherein said act ofreducing envelopes comprises minimizing of power consumption.
 28. Themethod according to claim 12, comprising the further act of: storing anadjusted phase shift value for each one of a set of used frequencies.29. The method according to claim 28, comprising the further act of:storing an adjusted phase shift value for each one of a set of usedfrequency generators for each of said used frequencies.
 30. Atransmitter unit, comprising: a first modulation unit configured toreceive a first digital signal and a first modulation selection signaland configured to output a first radio frequency signal corresponding tothe first digital signal modulated according to the first modulationselection signal; a second modulation unit configured to receive asecond digital signal and a second modulation selection signal andconfigured to output a second radio frequency signal corresponding tothe second digital signal modulated according to the second modulationselection signal; a first power amplifier operatively connected to thefirst modulation unit and configured to amplify the first radiofrequency signal; a second power amplifier operatively connected to thesecond modulation unit and configured to amplify the second radiofrequency signal; a combiner operatively connected to the first and saidsecond power amplifiers and configured to combine the first and saidsecond radio frequency signals and output the combined radio frequencysignals to a radio transmitter; and a power meter configured to measurea power level of the combined radio frequency signals from the combiner,wherein the first and said second modulation units are each operable toapply a modulation scheme according to the first and said secondmodulation selection signal, respectively, on a time slot basis, andwherein the first modulation unit comprises: a first modulation selectorconfigured to select one of a constant-envelop modulation scheme and anon-constant-envelop modulation scheme based on the first modulationselection signal; a first constant-envelop modulator configured tomodulate the first digital signal according to the constant-envelopmodulation scheme and output fist constant-envelop I and Q signals whenthe constant-envelop modulation scheme is selected; a firstnon-constant-envelop modulator configured to modulate the first digitalsignal according to the non-constant-envelop modulation scheme andoutput a first non-constant-envelop I and Q signals when thenon-constant-envelop modulation scheme is selected; a first separatorconfigured to separate the first non-constant-envelop I and Q signalsfrom the first non-constant-envelop modulator and into first component Iand Q signals and second component I and Q signals; a phase shifterconfigured to receive the first constant-envelop I and Q signals fromthe first constant-envelop modulator and the first component I and Qsignals from the first non-constant-envelop modulator and output phaseshifted I and Q signals based on the power level of the combined radiofrequency signals measured by the power meter; and a first analoguesignal generator configured to receive the phase shifted I and Q signalsfrom the phase shifter and the first constant-envelop I and Q signalsfrom the first constant-envelop modulator and output a first mixedsignal at a first carrier frequency, wherein the first mixed signal isprovided to the first power amplifier.
 31. The transmitter unit of claim30, wherein the first and said second component I and Q signals from thefirst separator are first-first and first-second component I and Qsignals, respectively, wherein the second modulation unit comprises: asecond modulation selector configured to select one of theconstant-envelop modulation scheme and the non-constant-envelopmodulation scheme based on the second modulation selection signal; asecond constant-envelop modulator configured to modulate the seconddigital signal according to the constant-envelop modulation scheme andoutput second constant-envelop I and Q signals when the constant-envelopmodulation scheme is selected; a second non-constant-envelop modulatorconfigured to modulate the second digital signal according to thenon-constant-envelop modulation scheme and output a secondnon-constant-envelop I and Q signals when the non-constant-envelopmodulation scheme is selected; a second separator configured to separatethe second non-constant-envelop I and Q signals from the secondnon-constant-envelop modulator and into second-first component I and Qsignals and second-second component I and Q signals; and a secondanalogue signal generator configured to receive the second-secondcomponent I and Q signals from the second separator and output a secondmixed signal at a second carrier frequency, wherein the second mixedsignal is provided to the second power amplifier.
 32. The transmitterunit of claim 31, wherein the second modulation unit does not include aphase shifting device.
 33. The transmitter unit of claim 30, wherein thesecond modulation unit comprises: a second constant-envelop modulatorconfigured to modulate the second digital signal according to theconstant-envelop modulation scheme and output second constant-envelop Iand Q signals when the constant-envelop modulation scheme is selected; asecond analogue signal generator configured to receive the secondconstant-envelop I and Q signals from the second constant-envelopmodulator and the second component I and Q signals from the firstseparator of the first modulation unit and output a second mixed signalat a second carrier frequency, wherein the second mixed signal isprovided to the second power amplifier.
 34. The transmitter unit ofclaim 30, wherein the first separator is configured such that a vectorsum of the first and said second component I and Q signals issubstantially equal to the first non-constant-envelop I and Q signalsfrom the first non-constant-envelop modulator.